[tzs]

Another possibility is that the Galactic Federation has a rule that when they need to visit a planet that is not yet aware of aliens the crew must entirely consist of beings that have the same general form as the people of that planet.

That makes it harder for someone who sees them to convince others that it was aliens and not just a trick of bad lighting or someone with deformities or injuries that give them an unusual appearance.

It also makes it less likely for them to be mistaken for some other species on the planet. Suppose the Federation sent an expedition to ancient Earth that included crew from a species that looked a lot like our cats, and accidentally let Earth people get too good a look at them and their technology.

Those Earth people might think those are Earth cats, and conclude that Earth cats are a lot more powerful and advanced then they thought, and that they had better stop treating cats like animals lest the cats decided to wipe them out and start treating them as superior beings. Next thing you know that entire civilization is treating cats as magical being of great value or even worshiping them as gods.

[HankB99]

Having owned (or more properly cared for) cats, I've always been a little suspicious.